Subsea pipelines have most often been laid on the ocean floor by connection or welding on the deck of a barge, lowering off the back of barge, and then laying on the ocean floor. The offshore pipeline industry uses various concepts of pipelaying.
A commonly known technique is referred to as the S-lay technique. In the S-lay technique, pipe sections are added to the previously launched pipeline in an assembly line, commonly including one or more welding station, which assembly line is arranged in a horizontal or near horizontal position on a pipelaying vessel, such as a lay barge. The pipeline then curves over the stern of the barge with the aid of a ramp or stinger, angles down toward the seabed, and curves back to horizontal and lays on the seabed. The launching of the pipeline is controlled by one or more pipeline tensioners, commonly positioned at the end of the assembly line and upstream of the stinger. These tensioners are capable of holding the previously launched pipeline. The profile of the pipeline from the pipelaying vessel to the seabed is in the form of a long “S”. The S-lay technique is mostly used in combination with relatively shallow water depths.
Alternatively, a vertical or near vertical laying techniques have been developed, primarily for deepwater offshore pipelaying, in particular the J-lay technique. J-lay pipelaying is conducted from a vessel having a tower mounted on the vessel. In a known arrangement individual pipe sections are loaded into the tower with a pipe loader and then positioned above the upper end of the previously launched pipeline. A welding station is commonly present to perform welding operations in order to connect a pipe section (or a subassembly of pipe sections) to the previously launched pipeline. In the tower, commonly below the welding station, a clamp is provided for holding and supporting the weight of the previously launched pipeline. After welding the pipeline and its newly-added pipe section(s) are lowered through this clamp and then clamp is again engaged with the pipeline near the upper end thereof. These steps are repeated as the vessel moves forward to thereby effect laying of the pipeline on the bottom of the sea. The launched pipeline follows a trajectory which has shape of a “J”, which “J” will not normally be vertical at the top, but rather has an angle with a horizontal component to be able to control the bending forces in the pipeline as it is touching the seabed.
It is also known to employ a “pipe storage reel” or “reel laying” method for laying an offshore pipeline. In this method the pipeline to be laid is manufactured in a continuous length on board of the pipelaying vessel or at a shore facility and then spooled onto a large spool. During the pipelaying process the pipeline is commonly straightened and passed over an (inclined) ramp or a tower. Tensioners and/or clamps are used for holding the previously launched pipeline. This method is mostly used with pipelines of limited diameter.
In most offshore pipelines to be laid on the seabed there is a need to install modules (accessories) on the ends of the pipeline and/or at one or multiple spaced intermediate locations along the pipeline. These modules are usually quite large and heavy and are commonly designed to be welded onto the pipeline. Examples of such modules are initiation fittings, pipeline end terminations (PLET), branches, inline SLED assemblies, inline manifolds, pipe valves, tee assemblies with their supporting structures and mud-mats.
Such pipeline accessories should often be welded inline with the pipeline. Often the accessory has pipeline connection parts at opposed ends thereof to connect the pipeline to those pipeline connection parts. It is however also known to have a pipeline accessory which is to be mounted, mostly by welding or using bolts, externally on the pipeline.
Various prior art solutions are known to mount an accessory at the end of a pipeline or at an intermediate location in a pipeline.
In WO 03/067019, a system for introducing an inline accessory into a pipeline is described. The system comprises a reel, a tensioner and a clamp, usually referred to as “hang-off” clamp. The clamp is adapted to support the weight of the launched pipeline which is suspended from said clamp. Furthermore, this known system comprises a ramp, here having variable inclination, with a pipeline feeding position and an accessory connection position, arranged next to one another. The area in between the tensioner and the clamp can be used to install accessories into the pipeline. The clamp is capable of translating horizontally—while the launched pipeline is suspended therefrom—from the pipeline feeding position to the accessory connection position. Means are provided for lowering the pipeline and accessory connected thereto past the clamp.
In US 2002/0159839 a pipelaying method and apparatus are described for near vertical laying of an offshore pipeline. The apparatus includes a tower and a hang-off clamp for suspending the launched pipeline. It is described that after the accessory has been welded to the pipeline in the tower, the clamp with the launched pipeline still suspended from the clamp and the accessory welded on top of the pipeline is moved generally horizontally to a position out of the tower. The accessory and launched pipeline are then connected to a hoisting cable of a crane mounted on the vessel. The weight of the launched pipeline and accessory is then transferred to the crane, whereafter the clamp is released from the pipeline. Hereafter, the clamp is displaced back in the tower and the crane then lowers the accessory and the launched pipeline connected thereto over predetermined height, essentially to below the tower. With the aid of the crane and the hoisting cable the accessory and the launched pipeline connected thereto are then returned to their original position in the tower where the clamp can engage the pipeline again above the accessory.
These known techniques for inserting an accessory into or at the end of a pipeline while laying an offshore pipeline are unsatisfactory.